1. Field of the Invention
The present invention relates to nonlinear dielectric elements (NLDE), and in particular ferroelectric ceramic capacitors utilized in high voltage pulse generating circuits for the ignition of discharge lamps or the like. High voltage pulses can be generated using a simple circuit consisting of a ferroelectric ceramic capacitor in series with a standard lamp ballast or an inductor having a good high frequency response. The discharge lamp is placed in parallel with the capacitor or NLDE. The pulse generating capability of the circuit is due to the polarization hysteresis loop generated by the NLDE in its ferroelectric state.
Each time the capacitor is driven into polarization saturation, a voltage pulse is created. The series inductance of the lamp ballast amplifies the pulse created by the polarization saturation, but because the impedance of the capacitor is much greater than that of the inductance, most of the pulse voltage appears across the capacitor. These voltage pulses can then be used to ignite specific types of discharge lamps connected in parallel with the capacitor.
The magnitude of the generated pulse is directly proportional to: (1) the remanent charge, i.e., the polarization value multiplied by the electrode area; (2) the slope of the polarization versus electric field response curve near the coercive field points, i.e., loop squareness; and (3) the applied voltage. Higher remanent charge levels, due either to increases in electrode area or higher remanent polarization levels, result in larger peak pulse voltages. Similarly, squarer polarization versus electric field behavior causes a greater change in the displacement current when the NLDE is driven into polarization saturation, also resulting in larger peak pulse voltages. Since the applied voltage cannot be varied in a standard lamp circuit, the first two factors will determine how well the NLDE will perform under specific temperature and voltage conditions.
The pulse behavior of the circuit therefore primarily depends upon the polarization behavior of the non-linear dielectric element. This performance can be improved by improving the size and shape of the polarization loop generated by the NLDE. The most significant factors defining the hysteresis behavior of the NLDE are its remanent polarization level, coercive field level and loop squareness. In addition the device should be capable of operating at as high a temperature as possible, and should be capable of withstanding the pulse that it generates without suffering dielectric breakdown. The polarization loop characteristics and other associated dielectric properties thus define the applicability of various ferroelectric materials for lamp ignitor applications. To be useful, the ferroelectric device should have a low coercive field value, a high remanent polarization, high dielectric strength, a high curie temperature and very square polarization versus voltage loop shape.
2. Description of the Prior Art
Prior nonlinear dielectric elements have been based either on BaTiO.sub.3 modified with up to 15 m/o of BaZrO.sub.3 or BaSnO.sub.3 or on Pb(ZrTi)O.sub.3 modified with BaTiO.sub.3 or SrTiO.sub.3. Both materials systems have significant shortcomings.
In the case of the modified BaTiO.sub.3, the Curie temperatures are limited to 90.degree. C. and the maximum remanent polarization level is limited to 0.20 C/m.sup.2 by intrinsic material characteristics. In spite of these limitations, very high voltage pulses can be generated below 70.degree. C. using a Ba(TiO.sub.0.915 Zr.sub.0.085)O.sub.3 -based composition. The pulse voltage level is decreased by 50% and the range of operation is extended to 85.degree. C. if the BaZrO.sub.3 content is decreased to 5 m/o.
Much higher temperature operating ranges are possible using (Pb.sub.1-x Ba.sub.x)(Zr.sub.1-y Ti.sub.y)O.sub.3 compositions. The Curie temperature range available in this system can be adjusted from above 300.degree. C. to below room temperature by adjusting the values of x and y. The major shortcoming of this materials system is the high level of coercive field inherent in PZT-based NLDE's, typically 10 KV/cm at room temperature. Materials having higher coercive field values have to be made thinner to allow their polarization loops to be opened by the applied voltage, and this reduced thickness makes them more susceptible to dielectric breakdown.